The present invention provides a two-stage melt polymerization process for the production of high molecular weight polybenzimidazoles utilizing certain dicarboxylic compounds as monomers.
High molecular weight polybenzimidazoles are polymers of high thermal stability and excellent resistance to oxidative or hydrolytic degradation, which can be formed into shaped articles such as fibers and films having very desirable physical properties. As taught by the published literature, these polymers may be prepared, for example, by melt polymerizing an aromatic tetraamine and a diphenylester or an anhydride of an aromatic or heterocyclic dicarboxylic acid in a one or two stage process; see, for example H. Vogel and C. S. Marvel, Journal of Polymer Science, Vol. L, pages 511-539 (1961); and U.S. Pat. Nos. Re. 26,065; 3,174,947; 3,509,108; 3,551,389; 3,433,772; and 3,655,632. In particular, U.S. Pat. No. 3,551,389 discloses a two stage process for the production of aromatic polybenzimidazoles, in which the monomers are heated at a temperature above 170C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product.
It has also been known to prepare polybenzimidazoles from the free dicarboxylic acids or the methyl esters of such acids rather than the phenyl esters or anhydrides in a melt polymerization process. However, many of the early investigators of these polymers believed that inferior or unsatisfactory polymers resulted from the use of the free acid or the dimethyl ester as the monomer. Thus Vogel and Marvel state in their cited article that in attempting to prepare polybenzimidazoles from aromatic dioic acids or their dimethyl esters and aromatic tetraamines, "The use of the free acids and the corresponding dimethy esters gave inferior results." (page 513) Moreover, these same two investigators do not mention the free acids or dimethyl esters as monomers in their U.S. Pat. No. 3,174,947 in which polybenzimidazoles are disclosed and claimed, nor are the free acids or dimethyl esters mentioned as possible monomers in any of the other cited patents.
In addition to the melt polymerization process described in the foregoing references, a process of preparing polybenzimidazoles has been proposed in which good results are reported to be obtained using a free dicarboxylic acid or a dimethyl ester of such acid as monomer. This process involves the solution polymerization of an inorganic acid salt of an aromatic tetra- amine and a dicarboxylic acid or a derivative thereof such as the dimethyl ester with heat in the presence of polyphosphoric acid; (see U. S. Pat. No. 3,313,783 and Iwakura et al, Journal of Polymer Science: Part A, Vol. 2, pages 2605-2515 [1964]). According to this process, the polymer product after completion of the reaction can be separated by pouring the reaction mixture in the form of a polyphosphoric acid solution into a large quantity of water. However, this separation procedure is complicated and, moreover, it is difficult to recover and reuse the polyphosphoric acid.
Despite the disadvantages of processes utilizing a free dicarboxylic acid or methyl, or other alkyl ester as monomer in the preparation of polybenzimidazoles, there are concomitant advantages to such use over the phenyl esters which are the most widely taught dicarboxylic acid derivatives for the preparation of these polymers. Thus, the free acid and alkyl esters are generally less expensive than the corresponding phenyl ester and their use does not result in the formation of free phenol as a by-product. Moreover, the formation of free phenol is undesirable because of toxicity problems and any expedient to eliminate or reduce such formation is considered beneficial. In view of this there have been attempts to develop processes which utilize a free acid as monomer and result in polymers which can be formed into shaped articles of commercial utility. For example, U.S. Pat. No. 4,312,976 discloses a single-stage process of preparing polybenzimidazoles from a free dicarboxylic acid as monomer, wherein the catalyst is an organic sulfonic acid, a halogenated acetic acid or a non-oxidizing inorganic acid. However, aside from being limited to a specific class of catalysts, this process results in a polymer mass of high viscosity requiring substantial agitation energy especially in the latter part of the polymerization process. Futhermore, the final polymer is in the form of a solidified mass which is difficult to handle and to dissolve in an appropriate solvent for forming into shaped articles such as fibers.
Copending application Ser. No. 645,952, filed Aug. 31, 1984, which is the parent of this application, discloses and claims a two stage process for the production of polybenzimidazoles using a free dicarboxylic acid as the dicarboxylic monomer component, which substantially eliminates the disadvantages of a one stage process utilizing the free acid described previously. However, this process as well as the previously known one stage processes usually results in the formation of water contaminated with tetraamine as a by-product which is difficult to dispose of. Thus, the replacement of at least part of the free acid with an alkyl ester, e.g. a methyl ester, results in at least part of the water being replaced by an alkanol such as methanol. This allows for easier disposal by combustion of the tetraamine-contaminated by-product.
Notwithstanding the disadvantages of the use of the phenyl esters and the advantages of the free acid and alkyl esters as previously pointed out, the use of a phenyl ester as part of the discarboxylic monomer component together with some free acid and/or alkyl ester may allow for the retention of some of the process advantages known to result from its use in one or two stage melt polymerization processes.